Paper is manufactured from cellulose fibers which may be extracted from wood or may be recovered recycled paper. The various sources and processes for creating and separating the individual wood fibers results in a paper stock containing contaminants which must be removed before the wood fibers can be used to make paper. While many contaminants can be removed from the fiber stock by washing, other contaminants are of a size or physical makeup which makes their removal by filtration difficult. Historically, hydrocyclones or centrifugal cleaners of relatively small size, normally from 2-72 inches in diameter, have been employed. It has been found that the centrifugal type cleaner is particularly effective at removing small size contaminants such as broken fibers, spherical particles, and seeds, as well as non-woody fine dirt such as bark, sand, grinderstone grit and metal particles.
The relatively small size of the centrifugal cleaners allows the employment of certain hydrodynamic and fluid dynamic forces provided by the combination of centrifugal forces and liquid shear planes produced within the hydrocyclone which allows the effective separation of small contaminants and debris.
The advent of certain modern sources of pulp fibers such as tropical wood species and recycled paper which is contaminated with stickies, waxes, hot melt glues, polystyrenes, polyethylenes, and other low density materials including plastics and shives presents additional problems in the area of stock preparation. The ability of the hydrocyclone to separate both high density and low density contaminants gives them particular advantages in dealing with the problem of cleaning modern sources of paper fiber. Many modern fiber sources tend to be contaminated with both heavyweight and lightweight contaminants.
In one common type of forward cleaner, the flow of acceptable material must change direction at the bottom of the cleaner and travel back up to the top. With such a cleaner in is difficult to effect changes in reject flow volume. To limit the amount of good fiber lost, it is necessary to restrict the volume of material rejected. This usually requires that the rejects orifice be small and in the center of the cleaner. Small orifices, however, are subject to clogging.
In my earlier U.S. Pat. No. 5,566,835 which is incorporated herein by reference, a hydrocyclone is described which can separate pulp stock into a heavyweight reject stream, a lightweight reject stream, and an accepts stream containing the useful wood fibers.
Through flows such as disclosed in the above referenced patent can develop a channeling of the injected flow which causes the injected flow to spiral down the inside surface of the cone forming the body of the hydrocyclone. This channeling limits the efficiency of the separation process.
While existing hydrocyclones have been developed to remove both heavy and light contaminants, further improvements in this area are highly desirable. The hydrocyclone as it is used to clean pulp is a small device, and is used in banks of up to sixty or more cleaners. Thus each hydrocyclone must be of extremely high reliability and require minimal maintenance or the entire hydrocyclone system will have poor reliability and high maintenance costs. Of particular relevance is the efficiency with which the hydrocyclone performs the separation function. Efficiency determines the number of stages which must be used to achieve a given level of separation. More separation stages means higher energy consumption and higher equipment costs.
What is needed is a through flow cleaner which is not subject to channeling thus providing increased effectiveness in separating desirable fiber from undesirable lightweight, and heavyweight components of a flow of pulp fiber stock.